Conventional vehicle suspension dampers typically consist of direct double-acting telescopic hydraulic dampers and are generally described as either shock absorbers or struts. This basic arrangement has been successfully used for well over half a century. A primary purpose of shock absorbers is to dampen oscillations of the vehicle's suspension springs. This is accomplished by converting kinetic energy in the form of motion between the sprung and unsprung masses of the vehicle into heat and then dissipating the heat. Struts also serve this capacity and in addition, act as a structural member to support reaction and side load forces on the suspension system.
Twin-tube dampers that provide a reservoir between the cylinder tube and a reservoir tube are well known. To house the twin-tube damper, a circular cylindrical reservoir tube having a closed bottom portion is generally provided. The top end of the circular cylindrical tube generally includes an opening through which the piston rod extends, and is typically provided with a means of sealing the area of the opening around the piston rod. Various techniques are presently used to close the ends of the circular cylindrical tubular sections for suspension dampers. One method is to weld a bottom closure to the tube. Another involves roll forming the wall of the tube to produce an integral closure.
Suspension dampers have evolved over the years, with the valving and rod guide components taking innumerable configurations and present day dampers are known in passive, adaptive, semi-active and active forms. Over the same period, the cylinder tube and, in the case of a twin-tube damper, the cylinder tube and the reservoir tube, have remained steadfastly unchanged. For example, U.S. Pat. No. 2,025,199 issued Dec. 24, 1935. That patent shows a direct acting shock absorber with a cylinder comprising "concentric" circular tubular members. U.S. Pat. No. 5,638,927 issued Jun. 17, 1997 and likewise shows a suspension damper with circular cylindrical cylinder and reservoir tubes. Accordingly, it appears the art has determined that circular cylindrical cylinder and reservoir tubes are optimal.
It is known that the bending moment of inertia for a circular cylindrical reservoir tube of a certain thickness is defined by the equation: ##EQU1## where D is the outside diameter of the tube, and d is the inside diameter of the tube. This bending moment of inertia defines the resistance to bending of the tube of a circular cross section about its center axis. The bending moment of inertia is an important concept since it defines the damper's ability to withstand operational induced stresses.
Typically, damper tubes are produced from roll formed sheet metal stock with welded mating edges, although the tube can also be impacted, drawn or extruded into a circular cylindrical form. This produces a convenient reservoir structure to facilitate end closure and for receiving the circular cylindrical cylinder tube. Since the top and/or bottom end closures are typically rolled closed, the circular cylindrical shape is advantageous. Typically, operational reciprocation of the piston within the cylinder tube results in some flexure induced rotation of the piston relative to the cylinder tube. Accordingly, the circular cylindrical shape advantageously accommodates the real world conditions.
Depending on piston rod size and reservoir size, a conventional damper may suffer undesirable oil starvation conditions. This can be complicated by a typical 10-15% oil loss over the extended life of an original equipment damper. During an extension cycle of a damper, oil is drawn into the working cylinder from the reservoir. Under oil starvation conditions, air may be drawn in along with the oil resulting in problematic cavitation conditions. Accordingly, a sufficiently large sized reservoir must be provided to avoid these conditions. Often the need to provide a larger reservoir conflicts with packaging constraints placed on the damper by the application. This leads to a need for greater flexibility in designing dampers for given applications than is presently available.